[Boost-users] Null shared pointer confusion

Subject: [Boost-users] Null shared pointer confusion
From: Carl Hetherington (lists_at_[hidden])
Date: 2009-07-08 19:23:31

Hi,

In the following program...

#include <boost/shared_ptr.hpp>
#include <boost/weak_ptr.hpp>

class A {};

int main ()
{
         boost::shared_ptr<A> x; // A: bad_weak_ptr thrown
//	boost::shared_ptr<A> x ((A *) 0); // B: works

         boost::weak_ptr<A> y (x);
         boost::shared_ptr<A> z (y);
}

... I observe that with line A included (but not line B), a bad_weak_ptr 
is thrown by the line which declares z.

With line B included (but not line A), all is well.

>From looking at the boost source, it seems that line A results in a 
shared_ptr to nothing (use_count() == 0), and line B results in a 
shared_ptr to 0 (use_count() == 1).

I can work around this, I'm sure.  Can anyone clarify my muddled thinking 
and explain why boost is behaving correctly in this situation (if indeed 
it is!)

Many thanks

Carl